Trip curves for motor overload relays are set at two meaningful calibration points: the ultimate trip current and the trip time at a specified current referred to as locked rotor current. The locked rotor current corresponds to a current required to overcome a locked rotor condition of a motor to cause the motor to begin to rotate. It is desirable for solid-state overload relays that implement a range of curves for different motor full load currents (MFLCs) to compensate for CT error (error in the current transformer(s) that monitor the current going through the relay) that differs across the range of measurement, compounded by the range of adjustment. An MFLC corresponds to a maximum current drawn by a motor when delivering its full rated output power (e.g., as specified by the manufacturer).
One approach is to characterize the CT error and use an error function to correct the input to a thermal model (i.e., a model that characterizes an overload trip curve in an overload relay). However, some overload relays, such as self-powered overload relays, have a limited amount of current available for powering the electronics, and performing this compensation digitally requires substantial processing power. While the error function can be implemented in an analog circuit, analog circuits require expensive, physically bulky, and current-consuming components.
Basic overload relays use a gain setting that compensates statically for the CT error. One implementation sets an adjustable gain based on the ultimate trip current setting to set the trip curve at the ultimate trip current. For each trip curve in the overload relay, a single gain is applied to the measured CT signal prior to input to a thermal model. But, this implementation cannot correct for changes in the CT error across the range of current covered by the trip curve. Depending on the variation of CT error, this method does not allow the overload relay to meet the required calibration points of the trip curve. Specifically, this method does not compensate for changes in CT error in the trip time between the ultimate trip current and the locked rotor current.
What is needed is a way of compensating for the variation in CT error across the range of currents protected by a trip curve for an overload relay.